configure: Do not unconditionally add -D_POSIX_C_SOURCE to CPPFLAGS.

Adding _POSIX_C_SOURCE to CPPFLAGS globally produces all sorts of problems
since it causes certain system functions to be hidden on some (BSD) systems.
The solution is to only add the flag on systems that really require it, i.e.
glibc-based ones.

This change makes BSD systems compile out-of-the-box without the need for
adding specific flags manually.  It also allows dropping a number of flags
set manually on a file-per-file basis, but were only present to work around
breakage introduced by the presence of _POSIX_C_SOURCE.

Also add _XOPEN_SOURCE to CPPFLAGS for glibc systems.  We use XSI extensions
in several places already, so it is preferable to define it globally instead
of littering source files with individual #defines only needed for glibc.